1. Field of the Invention
The present invention relates to random access channel (RACH) preamble detection in a communication system.
2. Description of Related Art
Expanded efforts are underway to support the evolution of the Universal Mobile Telecommunications System (UMTS) standard, which describes a network infrastructure implementing a next generation Wideband Code Division Multiple Access (W-CDMA) air interface technology. A UMTS typically includes a radio access network, referred to as a UMTS terrestrial radio access network (UTRAN). The UTRAN may interface with a variety of separate core networks (CN). The core networks in turn may communicate with other external networks (ISDN/PSDN, etc.) to pass information to and from a plurality of wireless users, or user equipments (UEs), that are served by radio network controllers (RNCs) and base transceiver stations (BTSs, also referred to as Node Bs), within the UTRAN, for example.
Setting up a communication channel typically involves the UE transmitting a known sequence, such as a sequence containing a series of symbols, on an access channel that is monitored by a receiver at the Node-B. The Node-B receiver detects the known sequence and uses it for functions such as estimating the round-trip delay between the UE and Node-B.
In UMTS, a random access transmission procedure may be employed to enable multiple UEs to share the same physical resources in establishing communications with a Node-B of a given cell. The Random Access Channel (RACH) is a common uplink transport channel that carries one or more preamble sequences and one or more message parts. In order to establish a connection for communicating with a Node-B in a particular cell (and the RNC serving the Node-B), a UE transmits the RACH on the uplink over a Physical Random Access Channel (PRACH) in the physical layer. The RACH is thus mapped to the PRACH.
The random access transmission may be based on a Slotted ALOHA approach with fast acquisition indication. In Slotted ALOHA, a UE may initiate the random access transmission at the beginning of a number of well-defined time intervals, known as access slots. There are 15 access slots per two frames. The access slots may be spaced 5120 chips apart. Information on what access slots are available for random access transmission may be given by higher layers (e.g., OSI layers 3-7).
The structure of the random access transmission includes a RACH preamble transmission followed by message part transmission. Each RACH preamble transmission is 4096 chips long and typically consists of 256 repetitions of length 16 Walsh-Hadamard preamble sequence signatures (denoted as ‘s’) that are carried on the PRACH, hence 16 preamble signatures. RACH preamble transmission(s) may be repeated with power ramping, e.g., increasing the preamble transmission power by a power ramping step size as signaled by the Node B, until the UE detects a downlink Acquisition Indicator Channel (AICH) transmitted by the Node-B. Accordingly, initial uplink synchronization in UMTS between the UE and Node-B may be achieved by detecting the RACH preamble. After successful reception of AICH, the UE may transmit a connection request within the message part of the PRACH. In response, the Node B sends a connection setup message through FACH in the Secondary Common Control Physical Channel (SCCPCH). This completes a transition from what is referred to as a UTRAN Idle Mode to a UTRAN Connected Mode (e.g., connection is established).
RACH preamble detection is done at the Node B receiver by correlating the received signal by a scrambling code and a signature sequence. Since a received preamble signal is delayed by the round-trip propagation time between the Node B and the UE, with respect to Node B transmit time, correlation is searched over a time range, or search window corresponding to the round-trip delay between the Node B and the UE. A preamble is detected and the signature sequence corresponding to a transmitted signature is found when the correlation energy exceeds a certain predefined threshold. Typically, the resolution of time search is coarse, i.e., at a half-chip resolution.
For preamble detection, a conventional receiver at a Node-B uses a single antenna, referred to as a ‘V-1’ antenna configuration, or two-diversity antennas, known as a ‘DIV-2V’ antenna configuration. Additionally, conventional preamble detection within the receiver at the Node-B may employ 4096-chip coherent integration to detect the preamble signature. A conventional preamble detector implementation is shown in 3GPP TSGR1 #6 (99) 893, entitled “Proposal for RACH Preambles.” A segmented preamble detector structure using sub-correlations instead of 4096-chip coherent integration may also be employed when the Doppler spread of the received signal is high, as is described in co-pending and commonly assigned U.S. patent application Ser. No. 09/665,511, filed Sep. 19, 2000 by Lee et al. and entitled “Segmented Correlator Architecture For Signal Detection In Fading Channels,” and as described in co-pending and commonly assigned U.S. patent application Ser. No. 09/664,646, filed Sep. 19, 2000 by Lee et al. and entitled “Segmented Correlator Architecture For Multiple Signal Detection and Identification In Fading Channels”.
At the Node B receiver, delay between the Node-B and the UE may be estimated by detecting one of the 16 preamble signatures. An initial search window to detect one or more of the preamble signatures may correspond to a round-trip delay between the Node-B and UE. Resolution of this initial search window is coarse. Typically, resolution of the initial search window is performed at half-chip resolution.
One issue being addressed by the 3rd Generation Partnership Project (3GPP), a body which drafts technical specifications for the UMTS standard and other cellular technologies, includes devising a procedure for determining a “best cell portion” during the random access transmission procedure described above between a UE, Node-B and serving RNC, as the UE attempts to establish a connection with the Node-B. A “best cell portion” may be understood as the portion of a cell where a received uplink signal has the highest signal to interference ratio (SIR). Beamforming using multiple antennas at Node B receiver may improve radio performance. Moreover, employing beamforming at the Node-B may also aid in determination of best cell portion, a measurement which may be included as part of the eventual Release 6 standard for beamforming that is still in development, 3G TR 25.887, V1.3.0 (October 2002), entitled “Beamforming Enhancements (Release 6)”.
Beamforming antennas represent an array of antennas used to form one or more beams within a cell having controlled beam directions. Beamforming modes may be defined as a flexible mode or a fixed mode. The flexible beamforming mode includes beamforming antennas where the uplink and downlink beams are formed by the application of weight vectors to the received and/or transmitted signals, in order to control the relative phase between the signals applied at the antenna elements. The weight vectors, and hence beam directions, are flexible. Beamforming with a grid of fixed beams (e.g., fixed mode) may be defined as beamforming antennas where the uplink and downlink beams are formed in such a way that the beam directions are fixed.
The best cell portion determination may be considered a beam-specific type of Node-B measurement. Beam specific Node-B measurements are intended for radio resource management (RRM) purposes such as admission control (AC), packet scheduling (PS), etc. During the random access transmission procedure, regardless of whether the beamforming mode is flexible mode or fixed mode, the RNC serving the Node-B should know in which beam direction a new UE is located. This information may be necessary to make a decision on whether the UE can get a call accepted. However, conventional algorithms for preamble detection in Node-B receivers having a ‘V-1’ or ‘DIV-2V’ antenna configurations, as described above, do not offer an approach to determining a best cell portion. Moreover, there is no methodology for detecting RACH preamble efficiently so as to establish uplink synchronization, or a connection, between a UE and a Node-B employing multi-antenna beamforming arrangements.